My Book 3.0 for Mac--cant download software on full disk!

I hope someone can help. I’m using a mid-2012 MacBook air with Mojave OS 10.14.6 and something is wrong with my computer disk. It says storage is full, and the operating system size is over 80 gb, so there’s something wrong right there. I erased everything I could from computer but want to save my pics so I thought I’d put them on My Book for Mac 3.0 that I have.

Anyway I can’t even install My Book for Mac software because there is no room on my MacBook disk. And I can’t delete anything else but I need to use an external hard drive in order to get the photos etc. off my Mac book air disk. So I’m in a conundrum.
I know this is old stuff, but I have 3 other WD external hard drives that won’t work either. Is there anyone familiar with older Airs that can help me figure out how to 1. get stuff off my Mac Book Air full disk
2. install external HD software on a full disk
Tricky stuff for an expert, which I am not. Thanks in advance.